Integrated directional coupler within an rf matching network

ABSTRACT

A directional coupler utilizes an inductive element of a power amplifier and a coupled conductive element. The inductive element of the power amplifier is a functioning element within the power amplifier and at least part of the inductive element of the power amplifier is disposed in a multi-layer substrate. At least part of the coupled conductive element is disposed in the multi-layer substrate. The coupled conductive element is configured to be inductively coupled to the inductive element of the power amplifier such that the coupled conductive element carries a first RF signal that is representative of a second RF signal within the inductive element of the power amplifier.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of application Ser. No. 13/841,278,filed Mar. 15, 2013, which claims the benefit of provisional applicationNo. 61/637,238, filed Apr. 23, 2012, entitled “Integrated DirectionalCoupler Within an RF Matching Network,” all which are incorporatedherein by reference.

BACKGROUND

Field of the Invention

This invention relates to RF systems and more particular to directionalcouplers.

Description of the Related Art

The physical implementation of radio frequency (RF) informationtransmission systems requires the amplification of the RF signal beforeit is injected into the communication channel. Examples of communicationchannels are free space in conjunction with antennas, coaxial cables,and wave guides. The amplification of the RF signal is provided by theRF power amplifier (PA). Many systems that include an RF power amplifierrequire sensing the amount of power generated at the output port of thePA for multiple reasons, e.g., power level control by way of feedback,reliability, and safety. A common approach to satisfying the requirementspecified above is the addition of a directional coupler insertedbetween the RF PA output and the point of injection of the RF signalinto the communication channel. The directional coupler siphons a verysmall but predictable portion of the RF signal destined for thecommunication channel and presents it to a dedicated port, the coupledport, where it is evaluated by other subsystems present in the system.

A directional coupler is an RF component typically comprising foursignal ports: an input port, an output port, a coupled port, and anisolated port. FIG. 1 shows a portion of an RF system according to priorart. The output 9 of the RF power amplifier block 10 is coupled directlyto the input port of the directional coupler. The output port of thedirectional coupler, node 3, is coupled to the input of thecommunication channel illustrated in FIG. 1 as resistor 6. Thecommunication channel input impedance is usually the characteristicimpedance of an RF transmission line, customarily 50-ohm. Thedirectional coupler of FIG. 1 has the coupled port shown as node 4 andthe isolated port shown as node 5. The isolated port, node 5, isconnected to resistor 7, typically 50-ohm, illustrating either aphysical resistor or in general terms, any port that presents animpedance appropriate for the described connection. When the PA injectsRF signal power on node 9, a fraction, defined as the coupling ratio, ofthe RF power traveling towards the load 6, typically −20 dB (1%),appears at the coupled port 4. The directional coupler is designed suchthat the coupled port 4 presents a signal substantially representativeof the RF power traveling from the PA 10 towards the load 6, while theisolated port 5 presents a signal substantially representative of the RFpower traveling from the load 6 towards the PA 10. A significant figureof merit for the directional coupler is represented by the directivityof the coupler defined as the ratio of the power presented at thecoupled port to the power presented at the isolated port, in thepresence of a perfect impedance match at node 3.

As further shown in FIG. 1, the PA 10 includes an RF amplifier stage 1and a matching network 2. The matching network 2 plays the role of animpedance transformation network, which converts the relatively highload impedance (e.g., 50-ohm) into a lower impedance (e.g., 5-ohm) asseen by the output of the final RF amplifier stage 1. It is commonpractice in the art to design the matching network to expect a loadimpedance on node 9 of 50-ohm. It is also common practice in the art todesign the directional coupler 8 to expect a load impedance on node 3 of50-ohm and further present an impedance of 50-ohm at node 9 so as tosatisfy the 50-ohm expectation of the matching network 2. Moreover, thedirectional coupler is designed for 50-ohm impedances at the coupledport 4 and isolated port 5.

Various prior art embodiments have demonstrated the ability to constructthe three parts shown in FIG. 1 within a PA module that satisfies therequirements set forth above. However, both the best achievable moduleperformance and the smallest achievable physical dimensions of themodule implementation are hampered by the rigid system partitiondepicted in FIG. 1.

SUMMARY

Accordingly, an embodiment provides an integrated directional couplerthat includes an inductive element within a power amplifier to carry afirst radio frequency (RF) signal and a coupled conductive element tocarry a second RF signal. The inductive element of the power amplifieris part of a tuned matching network within the power amplifier. Thecoupled conductive element is configured to be electromagneticallycoupled to the inductive element of the power amplifier such that thesecond RF signal is representative of the first RF signal.

In another embodiment an integrated directional coupler includes aninductive element within a power amplifier carrying a first radiofrequency (RF) signal and a coupled conductive element carrying a secondRF signal. The coupled conductive element is configured to beelectromagnetically coupled to the inductive element of the poweramplifier such that the second RF signal is representative of the firstRF signal. The inductive element of the power amplifier has a firstcharacteristic impedance that is different than a second characteristicimpedance of said coupled conductive element.

In another embodiment a method of making a directional coupler includesforming at least a portion of an inductive element of a power amplifierin a multi-layer substrate, the inductive element being part of a tunedmatching network within the power amplifier. The method further includesforming a conductive element in the multi-layer substrate, theconductive element configured to be electromagnetically coupled to theinductive element such that the conductive element carries a secondradio frequency (RF) signal that is representative of a first RF signalwithin the inductive element.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings.

FIG. 1 illustrates a system with a directional coupler according toprior art.

FIG. 2 illustrates a system partition in accordance with an embodimentof the present invention.

FIG. 3 illustrates an RF power amplifier stage and associated outputmatch circuit.

FIG. 4 illustrates a first embodiment of an integrated coupler inaccordance with an embodiment of the present invention.

FIG. 5 illustrates another example of an integrated coupler of anembodiment of the present invention showing formation of the inductiveelement of the coupler.

FIG. 6 illustrates an example of an integrated directional coupler thatincludes at least one inductive element of a power amplifier and adirectional coupler trace in accordance with an embodiment of thepresent invention.

FIG. 7a illustrates an example of an integrated directional coupler thatincludes at least one inductive element of a power amplifier and adirectional coupler trace in accordance with an embodiment of thepresent invention.

FIG. 7b illustrates a top view of an example of a substrate metalarrangement that satisfies the flow direction imposed in FIG. 7a inaccordance with an embodiment of the present invention.

FIG. 7c illustrates a cross section view of an example of substratemetal arrangement that satisfies the flow direction imposed in FIG. 7ain accordance with an embodiment of the present invention.

FIG. 8 illustrates an example of an integrated directional coupleraccording to an embodiment of the present invention that includes atleast one inductive element of a power amplifier and a directionalcoupler trace.

FIG. 9 illustrates an example of an integrated directional coupleraccording to an embodiment of the present invention that includes atleast one inductive element of a power amplifier and a directionalcoupler trace.

FIG. 10a illustrates an example of a lumped model for a directionalcoupler.

FIG. 10b illustrates a schematic representation of the lumped model fora directional.

FIG. 11 illustrates how the choice of the coupling factor k affects theachieved coupling factor and directivity of the directional coupler.

FIG. 12 illustrates how the choice of the coupling factor k affects theachieved coupling factor and directivity of the directional coupler.

DETAILED DESCRIPTION

It is recognized in embodiments of this invention that a conceptualchange of system partition to the one illustrated in FIG. 2 is highlydesirable in that it will remove constraints freeing the designer tochoose embodiments capable of superior RF performance and smallerphysical dimensions than attainable otherwise. The relative freedom ofdesign when implementing the system shown in FIG. 2 will become apparentfrom the detailed description of the embodiments of the invention asfollows.

The design process of directional couplers is well documented in theart. There are multiple ways to approach the directional coupler design.One example is to approach the design from a lumped model approximationof the directional coupler, which provides very simple closed formequations for the component values. FIGS. 10a and 10b show an example ofa lumped model for a directional coupler named the C-M model, whichstands for Capacitance-Mutual inductance model. FIG. 10a showsfiguratively the directional coupler, wherein two conductive traces arein close proximity to each other so as to be capacitively coupled withcapacitance C and magnetically coupled with mutual inductance M. The toptrace is the main signal path through the directional coupler. Thebottom trace is the coupled trace of the directional coupler,terminating on the coupled port and the isolated port, which are shownas connected to the termination resistors R. The characteristicimpedance of the main signal path through the directional coupler isdenoted as Zo.

FIG. 10b shows the schematic representation of the C-M lumped model ofthe directional coupler. Ports 51 and 52 are the input port and theoutput port of the main signal path, respectively. Ports 54 and 53 arethe coupled port and the isolated port, respectively. The model furtherincludes inductors 55, 56, 57, and 58 and capacitor 69. Inductor 55 and57 are magnetically coupled to each other with the coupling factor k.Inductor 56 and 58 are magnetically coupled to each other with thecoupling factor k. Resistors 59, 60, 61, and 62 are connected to thefour ports of the directional coupler. In its simplest form, the C-Mmodel uses identical inductors and termination resistors. For thissimplified case the equations relating the component values to thedirectional coupler's characteristic impedance and expected terminationresistors are:

C=k/ωR

M=CRZo

The equations above show how the required characteristic impedance andtermination resistors eliminate all but one degree of freedom fordesigning the directional coupler. For example, the remaining degree offreedom can be chosen to be the coupling factor, k. FIGS. 11 and 12 showhow the choice of the k value strongly affects the achieved couplingfactor and directivity of the directional coupler. As a result, eventhough k offers a degree of freedom to the designer, acceptabledirectional coupler performance can only be achieved for a very narrowrange of K values, substantially constraining the design. Variousembodiments of the present invention offer greater flexibility in thatthe choice of characteristic impedance for the coupler can be optimizedin conjunction with optimization of the output matching network.

FIG. 2 shows the PA module system partition in accordance with anembodiment of the present invention. The RF amplifier stage 201 iscoupled to the input 208 of the combined matching network and couplerblock 202. It is known to those skilled in the art that matchingnetworks comprise a plurality of impedances, at least one being aninductor. Any passive components associated with the final stage of thepower amplifier can be considered part of the output matching network.Each matching network inductor presents with the opportunity to set upan electromagnetic coupling to a metal trace, henceforth referred to asthe coupled trace of the integrated directional coupler, presentingports, which will play the role of the coupled port and isolated port ofthe directional coupler of the prior art. In reference to FIG. 2, thecoupled trace ports are node 204 as the coupled port, and node 205 asthe isolated port. Furthermore, the output of the matching network, node203, is coupled to the RF load 206. The system of FIG. 2 eliminates therequirement to design and implement a directional coupler that presentsthe standard 50-ohm impedance towards the matching network of a PA.

FIG. 3 shows an example of an RF power amplifier stage and itsassociated output match circuit. The output match circuit includeselements 303, 304, 305, 306, and 311. The output 310 appears across load307, figuratively shown as a resistor. In this power amplifier, any ofthe inductors, 303, 304, or 311 could be used as the inductive elementof the integrated directional coupler. It should also be understood thata transformer comprises multiple inductors and therefore the inductiveelement to the integrated directional coupler could be an inductor thatforms either a primary or a secondary coil of a transformer.

An example of a first embodiment of an integrated coupler is shown inFIG. 4. A power amplifier is built partly in die 402 and partly inmulti-layer chip carrier substrate 401. The power amplifier could bebuilt using any suitable technology for power amplifiers, including butnot limited to CMOS or GaAs HBT. The multi-layer substrate could bebuilt using any suitable material, including but not limited toIntegrated Passive Device (IPD), ceramic, or either cored or corelessorganic FR4. The multi-layer substrate comprises dielectric layers 403,404, 405, and 406 and metal traces such as 407, 408, and 409. In thefigure die 402 is flip chip mounted to substrate 401 using pillars 413or solder bumps, however it should be understood that die 402 could alsobe wirebonded to substrate 401. The wire loops 407 and 408 form aninductor that is part of the output matching network of the poweramplifier and is designed to resonate with other passive components onthe die or in the substrate. The wire loop 409 forms the coupled traceof the integrated directional coupler. It has an isolated port on pad411 and a coupled port on pad 410 on the bottom of the substrate. Thelength of the coupled trace and its proximity to the inductive elementare chosen to give the desired coupling ratio. A longer trace and/or acloser trace will give a higher coupling ratio. The inductance of wire409 and its magnetic coupling and capacitance to wire 408 are carefullytuned to give the desired characteristic impedance and to provide gooddirectivity. The exact geometrical shape of the wire loops 407, 408, 409is significant for the electrical parameters of the design however avariety of shapes can be used to achieve a desired target RF performanceand embodiments of the invention are not limited to the circular shapeshown in FIG. 4. By way of example a square, rectangular or octagonalshape wire or even a straight wire can be used instead of circles. Theexact number of wire loops is significant for the electrical parametersof the design, however a variety of loop counts can be used to achieve adesired target RF performance and embodiments of the invention may havea count other than the count of two (2) shown in FIG. 4.

FIGS. 3 and 4 are intended to show that the integrated directionalcoupler of embodiments of the present invention utilizes an alreadyexisting inductor from within the PA to form the inductive element ofthe coupler. It should be understood that the present invention is notlimited to this topology of power amplifier. Any other topology of RFpower amplifier that contains inductors or transformers in its outputmatching network can be used as part of the invention.

FIG. 5 shows another example of an integrated coupler of an embodimentof the present invention wherein the inductive element of the coupler isformed from via stack 502, wire 503, and via stack 504. It originates onthe die, where it connects to circuit elements, and returns back ontothe die, where it connects to circuit elements. The coupled trace isformed from via stack 506 and wire 505 and connects to pad 507. Itoriginates on the die, where it connects to circuit elements, runs nearthe inductive element of the coupler with an inductance, magneticcoupling, and capacitance that are carefully chosen to give the rightcombination of coupling ratio and directivity. The embodiment of FIG. 5is an example where coupling is achieved through a combination ofvertical coupling between segments 503 and 505 and of lateral couplingbetween segments 504 and 506. It should be understood that an integratedcoupler of the present invention could also be formed using strictlylateral coupling by positioning two wires adjacent to one another on thesame layer of metal.

FIG. 6 shows an example of an integrated directional coupler of anembodiment of the present invention that includes at least one inductiveelement of a power amplifier and a directional coupler trace. Thedirectional coupler trace carries an RF signal that is representative ofthe RF signal within the inductive element of the power amplifier. Theinductive element of the power amplifier overlaps the directionalcoupler trace and the inductive element of the power amplifier has adifferent trace width and a different impedance than the directionalcoupler trace. In the figure, metal traces 601, 603, and 605 aredifferent representations of the same metal trace. Metal traces 602,604, and 606 are different representations of the same metal trace.Traces 601 and 602 are in a perspective view. Traces 603 and 604 are topviews of traces 601 and 602, respectively. Traces 605 and 606 show across sectional view along the sectioning plane AA of traces 603 and604, respectively. In this embodiment the coupled trace 602, 604, or 606is substantially narrower than metal trace 601, 603, or 605, where metaltrace 601, 603, or 605 are part of the matching network inductiveelement chosen to be employed as the inductive element of the coupler.Metal traces 601 and 602 reside on adjacent substrate layers and layparallel to each other for at least part of their respective length.

FIG. 7 shows an example of an integrated directional coupler of anembodiment of the present invention that includes at least one inductiveelement of a power amplifier and a directional coupler trace. Thedirectional coupler trace carries an RF signal that is representative ofthe RF signal within the inductive element of the power amplifier. Thedirectional coupler trace is sandwiched between two segments of theinductive element of the power amplifier. As illustrated in FIG. 7 a,the coupled trace 702 is sandwiched between metal trace 703 and metaltrace 704, wherein metal trace 703 and 704 are part of the inductiveelement of the directional coupler, and where traces 702, 703, and 704are located on three adjacent substrate layers. The RF currenttraversing traces 703 and 704 of the inductive element flows in the samedirection in both wires 703 and 704 as exemplified by the arrows in FIG.7 a. This is accomplished by providing a path, not shown in FIG. 7 a,that allows current exiting the figure at point 730 to return to thefigure at point 731.

FIGS. 7b and 7c show an example of substrate metal arrangement thatsatisfies the flow direction imposed in FIG. 7 a. FIG. 7b and FIG. 7crepresent two views of the same metal structure. FIG. 7b is the top viewand FIG. 7c is the cross section along the plane AA depicted in FIG. 7b. The inductive element comprises via 705, trace 706, via stack 707,trace 708, via stack 709, trace 710, and via stack 711 as shown in FIG.7 c. The same inductive element is shown in FIG. 7b as comprising via715, trace 716, via stack 717, trace 721, and via stack 722. All othermetal parts of the inductive element are hidden from view in FIG. 7 b.The coupled trace comprises via stack 712 and trace 713 as shown in FIG.7 c. The same coupled trace appears as via stack 723, trace 718, trace719, and via stack 720 in FIG. 7 b. Traces 718 and 719 are on the samelayer. Via stack 720 has no visible counterpart in FIG. 7 c.

FIG. 8 shows an example of an integrated directional coupler of anembodiment of the present invention that includes at least one inductiveelement of a power amplifier and a directional coupler trace. Thedirectional coupler trace carries an RF signal that is representative ofthe RF signal within the inductive element of the power amplifier. Theinductive element of the power amplifier overlaps two turns of thedirectional coupler trace. The coupled trace comprises two metal traces,806 and 808, sandwiched between metal traces 805 and 807, where metaltraces 805 and 807 are part of the inductive element of the directionalcoupler, and where traces 805, 806, 807 are located on three adjacentsubstrate layers and where traces 806 and 808 lie on the same substratelayer. The RF current traversing traces 805 and 807 of the inductiveelement is arranged so that it flows in a direction as shown by thearrows in FIG. 8. This is accomplished by providing a path, not shown inFIG. 8, that allows current exiting the figure at point 801 to return tothe figure at point 803 and a second path, not shown in FIG. 8, thatallows current exiting the figure at point 802 to return to the figureat point 804.

FIG. 9 shows an example of an integrated directional coupler of anembodiment of the present invention that includes at least one inductiveelement of a power amplifier and a directional coupler trace. Thedirectional coupler trace carries an RF signal that is representative ofthe RF signal within the inductive element of the power amplifier. Theinductive element of the power amplifier overlaps the directionalcoupler trace and one or more shields are located within the substrateto shield said directional coupler trace from undesired signals and totune the impedance present on the directional coupler trace. In FIG. 9,the coupled trace comprises via 901, trace 902, via 903, and pad 908.Pad 908 is connected to the coupled trace by means of a conductive pathelsewhere in the substrate and not shown in the figure. The inductiveelement of the PA comprises via stack 904, trace 905, via stack 906, andbottom pad 907. The metal structure 909 forms a shield, which reducesthe capacitive coupling between pad 908 of the coupled trace and segment905 of the inductive element. At the same time, the additionalcapacitance between pad 908 and shield 909 adjusts the tuning of thecoupled trace. These effects can be used to improve the directivity ofthe coupler.

The integrated directional coupler embodiments described above are shownto be built in a chip carrier substrate. That approach allows theinductive element and the coupled trace to both be built with high Q ina substrate and the size of these elements also makes them well suitedfor a substrate. However, it should be understood that an integrateddirectional coupler of an embodiment of the present invention could alsobe built on a power amplifier die. In such an embodiment at least partof the coupling between the inductive element and coupled trace wouldoccur on the die. The structure would otherwise be similar to thoseshown in the figures above.

Thus, embodiments have been described capable of superior RF performanceand smaller physical dimensions. The description of the invention setforth herein is illustrative, and is not intended to limit the scope ofthe invention as set forth in the following claims. Other variations andmodifications of the embodiments disclosed herein may be made based onthe description set forth herein, without departing from the scope ofthe invention as set forth in the following claims.

What is claimed is:
 1. An integrated directional coupler, comprising: aninput port; an output port; an isolated port; a coupled port; aninductive element within a power amplifier, wherein the inductiveelement is coupled between the input port and the output port, andwherein the inductive element includes a first wire loop; and a coupledconductive element coupled between the isolated port and the coupledport, wherein the coupled conductive element is electromagneticallycoupled to the inductive element such that a signal at the coupled portis representative of a signal output by the power amplifier, and whereinthe coupled conductive element includes a second wire loop.
 2. Theintegrated directional coupler of claim 1, wherein the first wire loopand the second wire loop are separated by a dielectric layer.
 3. Theintegrated directional coupler of claim 1, wherein the first wire loopand the second wire loop are circular.
 4. The integrated directionalcoupler of claim 1, wherein the first wire loop and the second wire loopare octagonal.
 5. The integrated directional coupler of claim 1, whereinthe inductive element further includes a third wire loop coupled inseries with the first wire loop.
 6. The integrated directional couplerof claim 5, wherein the first wire loop is separated from the third wireloop by a first dielectric layer and the second wire loop is separatedfrom the third wire loop by a second dielectric layer.
 7. The integrateddirectional coupler of claim 5, wherein the first wire loop is disposedon a first dielectric layer of a multilayer substrate, wherein the thirdwire loop is disposed between the first dielectric layer and a seconddielectric layer of the multilayer substrate, and where the second wireloop is disposed on the second dielectric layer of the multilayersubstrate.
 8. The integrated directional coupler of claim 7, wherein thepower amplifier is at least partially disposed in an integrated circuitdie mounted to an upper surface of the multilayer substrate.
 9. Theintegrated directional coupler of claim 8, further comprising: a firstpad coupled to the isolated port; and a second pad coupled to thecoupled port, wherein the first pad and the second pad are disposed on abottom surface of the multilayer substrate.
 10. The integrateddirectional coupler of claim 9, further comprising a third pad coupledto the output port, wherein the third pad is disposed on the bottomsurface of the multilayer substrate.
 11. The integrated directionalcoupler of claim 10, wherein a first end of the second wire loop iscoupled to the first pad by a first via, wherein a second end of thesecond wire loop is coupled to the second pad by a second via, whereinan end of the first wire loop is coupled to a first end of the thirdwire loop by a third via, and wherein a second end of the third wireloop is coupled to the third pad by a via stack.
 12. The integrateddirectional coupler of claim 1, wherein the first wire loop is disposedon a first dielectric layer of a multilayer substrate, and where thesecond wire loop is disposed on a second dielectric layer of themultilayer substrate.
 13. The integrated directional coupler of claim12, wherein the power amplifier is at least partially disposed in anintegrated circuit die mounted to an upper surface of the multilayersubstrate.
 14. The integrated directional coupler of claim 13, furthercomprising: a first pad coupled to the isolated port; and a second padcoupled to the coupled port, wherein the first pad and the second padare disposed on a bottom surface of the multilayer substrate.
 15. Theintegrated directional coupler of claim 14, further comprising a thirdpad coupled to the output port, wherein the third pad is disposed on thebottom surface of the multilayer substrate.
 16. A radio-frequencymatching network and integrated directional coupler, comprising: a tunedmatching network having an inductive element to carry a firstradio-frequency signal, the inductive element including a first wireloop; and a coupled conductive element to carry a second radio-frequencysignal, the coupled conductive element including a second wire loopelectromagnetically coupled to the first wire loop such that the secondradio-frequency signal is representative of the first radio-frequencysignal.
 17. The radio-frequency matching network and integrateddirectional coupler of claim 16, wherein the tuned matching network isan output match circuit of a power amplifier.
 18. The radio-frequencymatching network and integrated directional coupler of claim 17, whereinthe first wire loop is configured to resonate with at least one otherpassive component.
 19. The radio-frequency matching network andintegrated directional coupler of claim 16, wherein at least part of theinductive element is disposed in a multilayer substrate and wherein atleast part of the coupled conductive element is disposed in themultilayer substrate.
 20. The radio-frequency matching network andintegrated directional coupler of claim 19, further comprising adielectric layer disposed between the first wire loop and the secondwire loop.
 21. The radio-frequency matching network and integrateddirectional coupler of claim 19, wherein the inductive element furtherincludes a third wire loop coupled in series with the first wire loop.22. The radio-frequency matching network and integrated directionalcoupler of claim 21, wherein the first wire loop, the second wire loop,and the third wire loop are separated by dielectric layers.
 23. Theradio-frequency matching network and integrated directional coupler ofclaim 19, further comprising a power amplifier having an output coupledto the first wire loop.
 24. The radio-frequency matching network andintegrated directional coupler of claim 23, wherein the power amplifieris disposed in an integrated circuit die mounted to the multilayersubstrate.
 25. The radio-frequency matching network and integrateddirectional coupler of claim 16, wherein the first wire loop and thesecond wire loop are circular.
 26. The radio-frequency matching networkand integrated directional coupler of claim 16, wherein the first wireloop and the second wire loop are octagonal.